Many battery systems, such as the ubiquitous Li-ion battery, are extremely sensitive to water. These cells are enabled through the use of nonaqueous technologies which expand the operating potential windows of the cells significantly. Any water infiltration, even on the order of a few tens of ppm, has disastrous effects on the reliability and electrochemical performance of the cell within. In order to maintain the required purity, much attention is given to the packaging material which protects the electrochemical cell. Key to such cells is the application of a barrier between the cell and the ambient environment to prevent exposure of the cell to oxygen and moisture which would degrade performance of such cells drastically. Hermetic or near hermetic conditions are required. In order to supply such conditions, one typical state of the art packaging consists of a metal can, which is crimped closed with a polymer gasket or laser welded with glass metal seals to allow the current collectors through without shorting the cell. These cans have limited shape flexibility, considerable weight, and volume. Alternative packaging in the form of flexible multi laminate packaging consists of layers of distinct function. In short these packages consist of an inner layer of metal, typically Al of 20-60 microns, an outer layer of a tough mechanical barrier polymer such as a polyester, and an inner heat sealable layer consisting of an acid modified polyolefin (See, Jansen et al., Low-Cost, Flexible Battery Packaging Materials, JOM, 2002 March, pp. 29-32 and 54).
It is important that such barrier protection is not only effective at the face of the barrier but also at perimeters where the barrier is sealed to itself or to protruding tabs. Relative to the seals, the current materials found on today's state of the art batteries include acid modified polyolefins such as poly ethylene and poly propylene. A severe problem exists as these materials, although somewhat effective as a barrier to the permeation of moisture and oxygen, are very poor barriers to the permeation of linear carbonates commonly utilized in lithium batteries today, including, but not limited to, dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). Once these materials are swollen with these solvents, the barrier protection these materials offer to the battery cell degrades drastically. It would be desirable to have a material that has outstanding intrinsic barrier properties to the transport of solvents while affording the ability to seal the material to itself and also protruding metallic tabs. This will allow a significant decrease in the seal width and therefore allow improvement in volumetric efficiency of the battery.
Parylene is a material with known hydrophobicity and excellent barrier properties to the permeation of a number of chemical compounds. To our knowledge, Parylene has never been disclosed as a good barrier for non-aqueous batteries containing solvents with the ability to have self standing sheets, the ability and technique to seal to itself and to components of the battery such as tabs, and the use of multilayer composites to afford additional barrier properties.